1. Field of the Invention
The present invention relates to a magnetoresistive effect element (MR element) in a current perpendicular to plane (CPP) structure that detects magnetic field intensity as a signal from a magnetic recording medium, and so on, a thin film magnetic head with the MR element, and a head gimbal assembly and a magnetic disk device that have the thin film magnetic head.
2. Description of Related Art
In recent years, with an increase in the high recording density of magnetic disk drives (HDD), there have been growing demands for improvements in the performance of thin film magnetic heads. For a thin film magnetic head, a composite type thin film magnetic head has been widely used; it has a structure where a reproducing head having a read-only magnetoresistive effect element (hereinafter, magneto-resistive (MR) element) and a recording head having a write-only induction type magnetic conversion element are laminated together.
As a reproducing head, an MR element having a so-called current in plane (CIP) structure has widely been used. The MR element operates by allowing an electric current to flow in parallel with the film surface of an element referred to as a spin valve GMR element having a CIP structure (CIP-GMR element). The spin valve GMR element having such a structure is placed between upper and lower shield layers formed of soft magnetic metal films in a manner of being sandwiched between insulating layers referred to as gap layers. The recording density in the bit direction is decided by a gap (i.e., a shield gap or read gap length) between the upper and lower shield layers.
As the recording density increases, there is an increasing need for a narrower shield gap and track in a reproducing element of a reproducing head. As a result of the narrower track of the reproducing element and the reduction in the height of the element associated therewith, the area of the element decreased. A problem existed that the operating current was limited in the conventional structure in terms of reliability because the heat dissipating efficiency decreased as the area decreased.
In order to solve the above-mentioned problem, a GMR element having a current perpendicular to plane (CPP) structure (CPP-GMR element) has been proposed. The CPP-GMR element does not need an insulating layer between upper and lower shield layers (i.e., the upper part shield layer and the lower part shield layer) through electrically connecting the upper and lower shield layers to an MR element in series. This technology is essential to achieve a high recording density that exceeds 200 Gbits/in2.
The CPP-GMR element has a lamination structure containing a first ferromagnetic layer and a second ferromagnetic layer formed in a manner of sandwiching a conductive nonmagnetic intermediate layer from both sides. The typical spin valve type CPP-GMR element has a lamination structure from the substrate side sequentially as follows: a lower electrode, an antiferromagnetic layer, a first ferromagnetic layer, a conductive nonmagnetic intermediate layer, a second ferromagnetic layer and an upper electrode.
A magnetization direction of the first ferromagnetic layer, which is one of the ferromagnetic layers, is pinned in the perpendicular direction to a magnetization direction of the second ferromagnetic layer when the externally applied magnetic field is zero. The magnetization of the first ferromagnetic layer can be pinned by making an antiferromagnetic layer adjacent thereto and providing unidirectional anisotropic energy (also referred to as “exchange bias” or “coupled magnetic field”) to the first ferromagnetic layer by means of exchange-coupling between the antiferromagnetic layer and the first ferromagnetic layer. For this reason, the first ferromagnetic layer is also referred to as a magnetic pinned layer. On the other hand, the second ferromagnetic layer is referred to as a free layer. Moreover, the structure of the magnetic pinned layer (i.e., the first ferromagnetic layer) having a three-layer structure of a ferromagnetic layer/a nonmagnetic metal layer/a ferromagnetic layer (so-called “a synthetic ferrimagnetic (SyF) structure” or “a synthetic pinned structure”) allows not only providing strong exchange-coupling between the two ferromagnetic layers and effectively increasing the exchange-coupling force applied from the antiferromagnetic layer but also decreasing the influence of a static magnetic field generated from the magnetic pinned layer toward the free layer. Therefore, the “synthetic pinned structure” has widely been used lately.
Nevertheless, there remains a need for further thinning an MR element in order to meet the recent demand for extra high recording density. In such circumstances, novel GMR element structures have been proposed that have a simple three-lamination layer structure of a ferromagnetic layer (free layer)/a nonmagnetic intermediate layer/a ferromagnetic layer (free layer) as a basic structure as disclosed in U.S. Pat. Nos. 7,019,371 B2, 7,035,062 B1, and 7,177,122 B2, etc.
In this application, such a structure is conveniently referred to as a dual free layer (DFL) element structure. In the DFL element structure, magnetization directions of two ferromagnetic layers (free layers) are exchange-coupled in a manner of being antiparallel to each other. Moreover, a magnet is disposed on a posterior area opposite to an air bearing surface (ABS), which corresponds to an opposing medium surface of an element, and a predetermined state is created using an influence of a bias magnetic field generated from the magnet in which each magnetization direction of the two magnetic layers (free layers) tilts about 45 degrees relative to the track width direction (initial state).
At a time when an element in the initial magnetization state detects a signal magnetic field from a medium, the magnetization directions of the two magnetic layers change in a scissor-like action like a pair of scissors cuts paper. As a result, a resistance value of the element changes.
The application of such the DFL element structure to a so-called tunneling magnetoresistance (TMR) element or CPP-GMR element allows more narrowing of the “read gap length,” a gap between upper and lower shield layers, compared with a conventional general-type spin valve type CPP-GMR element. More specifically, the above-mentioned antiferromagnetic layer that is required for a general spin valve type CPP-GMR element is no longer needed. Nor is the ferromagnetic layer of the above-mentioned “synthetic pinned structure” needed any longer. As a result, it is possible to reduce the “read gap length,” the limit of which is conventionally believed to be 30 nm, to 20 nm or less.
In this DFL element structure, since the read gap length can be narrowed, it is possible that a high recording density in the track direction is realized. It is necessary that a high recording density in the track width direction is realized as well to provide an overall high recording density. The technology that enables narrowing of the width (in the track width direction) of an MR element itself has been developed. However, with respect to the high recording density of 660 Gbpsi or more in the future, even though the width of the MR element is narrowed, there was an indication that the substantial reading width is hard to be narrowed. When the conventional technology is used, the photo-width (or width of a pattern made with a photoresist) of 20 nm or less is required and it is very hard to manufacture in view of the process technology.
In consideration of the situation mentioned above, it is possible that side shield layers are formed at both sides of an element in the element width direction. Since, fortunately, a bias magnet, which makes magnetization directions of two free layers orthogonal each other, is provided in the rear side of the element in the above DFL structure, it is possible to arrange the side shield layers at both sides of the element in the element width direction. However, when the side shield layers are simply formed at the both sides of the element in the element width direction, a magnetic field in a cross direction to the element is applied due to a signal magnetic field from an adjacent track next to a primary track to be reproduced. When the magnetic field in the cross direction to the element (mainly the recording magnetic field of the adjacent track) is applied to the DFL element structure, a magnetization direction regulation state (an initial magnetization direction state tilting about 45 degrees relative to the track width direction (initial state)) of the two free layers is broken, i.e., a behavior balance is broken, and there is a possibility that the DFL element cannot operate properly.
In consideration of the situation described above, the present invention is provided. An object of the present invention is to provide an MR element realizing a further high density in the track width direction through the side shield effect while the magnetization direction regulation state of two free layers is held in the normal one in the so-called DFL element structure.